1. Field of the Invention
The present invention relates to a dental implant comprising a metal core and a layer of biologically active glass or glass ceramics covering the metal core, and also to a method of making the dental implant.
2. Description of the Prior Art
In dental prosthesis many attempts have been made to use a dental root generally called dental implant. When an artificial tooth is to be set for a decayed-out natural tooth, the implant is implanted in the alveolar bone and a dental crown is mounted on the implant. Although this method is very attractive, the conventional dental implants have an important problem. Because of the mechanical strength required for such dental implants they have to be made from those materials which have high mechanical strength such as metal. The bonding between the implant and the bone relies upon the mechanical bonding force (interlocking or anchoring) only. Relying on mere mechanical bonding force it has not always been possible to perfectly fix the implant in the bone. Due to such insufficient stabilization, there has been often caused inflammation in the alveolar bone after implanting. In a relatively short time of use the implant falls out. Because of these drawbacks it has been difficult to put such metal implants to practical use.
Recently, biologically active glasses or glass ceramics have been developed in the art which can directly and chemically combine with bones (cf. Japanese Patent Application laid-open No. 145,394/1978 the counterpart of which is U.S. Pat. Nos. 4,159,358 and 4,234,972). Such biologically active glass or glass ceramics is hereinafter referred to as bioglass. As an application of the bioglass it has been proposed also to make dental implants by covering a metal core with a bioglass layer.
The mechanism of the chemical bonding between bioglass and bone has not yet been fully explained. They say that under the action of body fluids or humor the atoms in the bioglass are dissolved out as ions which deposit in the area near the boundary between bioglass and bone and in some cases there are formed the same compounds as the inorganic compounds in the bone thereby forming a direct and strong bioglass-to-bone bond.
However, it has been found that the properties of the bioglass surface change gradually with the dissolution of ions whereby a layer is formed which has different properties from those of the initial bioglass. The layer may be called a reaction layer. We, the inventors of the present invention have made a vast study on the bioglass and the bonding strength of the bioglass and bone. The study has led us to the following findings:
The forming speed of the above mentioned reaction layer decreases as the bonding between the bioglass and the bone is nearing completion. However, if the reactivity of the bioglass is high, the dissolution of ions from the bioglass steadily continues for a long time although the rate of dissolution becomes lower and the reaction layer continues growing. At last the reaction layer develops up to the interface area between the metal core and the bioglass layer. In general, the reaction layer thus formed is, unfortunately, fragile. The mechanical strength is lower than the original bioglass and, as a matter of course, lower than that of the metal core. Therefore, when the reaction layer has grown too much and in particular when the reaction layer has developed up to the interface area between the bioglass and the core, the bonding strength between the glass and the metal core is reduced markedly. In the worst case, the implant falls out by occlusal pressure.
The above findings lead us to the conclusion that the growth of the reaction layer should be stopped before the reaction layer has developed into the interface area between bioglass and metal core and that the growth of the reaction layer should be stopped preferably in a gentle and gradual manner but not abruptly. An ideal bioglass to satisfy the desire is such glass which has high reactivity at the area near the interface between the glass and the alveolar bone to provide good bondability (good initial bondability) but has low or substantially no reactivity at the area near the interface between the glass and the core to provide good long-term bondability. However, as will be readily understood, a single kind of glass can not satisfy both of the initial bondability and the long-term bondability at the same time. To solve the problem, we have conceived a two-layer bioglass covering comprising an outer layer of high reactivity and an inner layer of low or substantially no reactivity.
The conception of the use of a multi-layer bioglass covering in dental implant is not novel per se. From the description made in the above-referred U.S. Pat. No. 4,159,358 we have been informed of the fact that the prosthetic device disclosed in German Pat. No. DT 2,326,100 B2 has been invented based on a conception similar to our conception of multi-layer bioglass covering described above. The prosthetic device disclosed in the German patent has an intermediate layer formed of less active glass and interposed between a metal core and a bioglass layer.
Generally, the simplest process for covering a metal core with glass is the melt immersion process. However, when this melt immersion process is employed, it is required that the metal core and glass should have the same thermal expansion coefficient. Therefore, if a metal core is immersed in molten glass having different thermal expansion coefficients from that of the metal core, then a serious problem is caused thereby. The glass coating layer formed on the metal core after it is removed from the immersion bath and allowed it to cool down to the room temperature has a large residual stress therein. Due to the large residual stress, the glass breaks or at least the glass is easily broken.
In the above-referred U.S. Pat. No. 4,159,358, Hench et al have proposed the following bioglass composition:
______________________________________ SiO.sub.2 40-60% (by weight) Na.sub.2 O 10-32% CaO 10-32% P.sub.2 O.sub.5 0-12% CaF.sub.2 0-18% B.sub.2 O.sub.3 0-20% ______________________________________
Within the above range of composition such a particular bioglass may be obtained which has the same thermal expansion coefficient as that of a selected biocompatible metal core, for example, that of a cobalt-chromium alloy, SUS 316 stainless steel. However, when such a particular bioglass composition is selected from the above, the biological activity of the glass is directly and exclusively determined by it. Therefore, as stated by Hench et al themselves, it was impossible to obtain implants having different biological activities employing the same metal core. In other words, when one attempts to coat a metal core with two glass layers having different biological activities, one can not select such a combination of two kinds of glass from the glass compositions proposed by Hench et al which are different from each other in activity but the same in thermal expansion coefficient.
For the reason described above, Hench et al gave up the attempt to match the thermal expansion coefficient of bioglass with that of metal core. Instead, they have invented a novel method of making implant (U.S. Pat. No. 4,159,358) which provides the above-mentioned type of implant even when the metal core and the bioglass then used have different thermal expansion coefficients.
However, there are some difficulties in manufacturing a dental implant according to the method proposed by Hench et al.
The metal core generally used in dental implants is very small in size. It is a conical metal member of 3-6 mm in diameter and 8-12 mm in length. Naturally its thermal conductivity is very high. The metal core is immersed in a molten glass mass of high temperature in the range of from 1250.degree. to 1550.degree. C. The essential thing for this process is to rapidly lower the temperature of the molten glass coating layer to a certain critical temperature (Ts) before the temperature of the metal core rises. The critical temperature (Ts) is the temperature at which the temperature dependence of the volume of bioglass becomes non-linear (refer to the patent specification of Hench et al). However, considering the very small size and very high thermal conductivity it is obvious that such rapid cooling is extremely difficult to realize. Generally speaking, a simpler manufacturing process is advantageous from the standpoint of manufacturing cost.
As previously noted, the melt immersion process is the simpliest process for coating a metal core with glass. However, in the case wherein a metal member is to be covered with two layers, an inner glass layer and an outer glass layer, the two glass layers are required to have the same thermal expansion coefficient. Otherwise, the two-layer glass coating formed on the metal core has a large amount of residual stress therein after cooling. Due to the residual stress, the glass coating breaks at once or is easily broken.
Furthermore, the glass for the inner layer is required to have a higher melting point than that for the outer layer. Otherwise, the inner layer melts and flows when the outer layer is applied on it, which results in deformation of the coating.